CHANGES in product durability disturb existing supply and demand relation- ships. Whether they occur ultimately depends on expected profits and, there- fore, on the degree of competition among sellers. In competitive markets the firm is constrained by the actions of other producers. Being slow to introduce an innovation may force it to leave the industry as consumers turn to the improved products of competitors. Will the monopolist, who faces no such immediate constraints, produce a different quality of product from that which would emerge from competitive markets? Specifically, will product durability be different under alternative industry structures? If so, will monopoly, competition, or some monopolistically competitive structure pro- duce the 'better' product?